You can use GNU Go to estimate the score of a game. To do this, select Guess score in the Edit menu of a board window. This will first save the game to a temporary file. The file location depends on your operating system. Then it will start a GNU Go process with something like this: “gnugo --score estimate --quiet -L 254 -l /tmp/glGoxfCodg”. This would estimate the score at move 254 in the given temporary SGF file.

The result will be displayed in a dialog once GNU Go has estimated the score. GNU Go might need some time for this, especially if the game is still in the middle game.